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146 E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 The rationale for operating a hydroformylation re- action in a supercritical fluid is similar to that for hydrogenation. Hydroformylation involves the use of two gaseous reactants (CO and H2) and hence hydro- formylation of a non-volatile or low volatility liquid substrate will likely be limited by the solubility and transport of the gaseous reactants from the vapor to the liquid phase. As for the case of hydrogenation in supercritical fluids, research on hydroformylation has been conducted using both homogeneous and hetero- geneous catalysts. The ‘green’ rationale for explor- ing this class of reactions using SCF solvents is that creation of a more efficient reaction (kinetically con- trolled, more selective) will result in the production of fewer byproducts and perhaps require lower en- ergy input. Given the conditions under which the pro- cess is currently operated, if one could produce the same space-time yield of product using lower pressure and/or temperature, the savings could be significant. In summary, the green premise behind conducting hydroformylation in CO2 is not only to replace solvent (only a factor in some oxo processes), but also to create a more efficient reaction, and hence reduce byproduct waste and energy input. 2.7.1. Homogeneous catalysis of hydroformylation in CO2 Rathke et al. [76] reported the hydroformylation of an olefin in CO2 in 1991. Here, a cobalt carbonyl catalyst (soluble in CO2 without modification) was used to promote the generation of butyraldehyde from propylene, CO2 and hydrogen. Rathke reported that operating the reaction in CO2 produced a somewhat improved yield of linear to branched aldehyde. The rate of formation of both cobalt intermediates and aldehydes was found to be similar to values found when the reaction was performed in conventional non-polar solvents. Leitner et al. [103], as well as Erkey et al. [104], reported hydroformylation of an olefin in supercrit- ical CO2 using a homogeneous rhodium catalyst in 1998, where the now classic strategy of derivatizing the catalyst ligands with fluorinated ponytails was used to enhance catalyst solubility. Leitner found that the reaction (hydroformylation of 1-decene) readily goes to completion in CO2, with catalyst activities simi- lar to those reported in liquid systems. Erkey’s re- sults for 1-octene are similar. As Leitner points out, the long-chain alkenes employed as substrates for the reactions in CO2 would likely not be soluble in wa- ter and hence the well-known aqueous Rh/triphenyl phosphine trisulfonate catalyst system cannot be used to generate long-chain aldehydes. Here, potentially, is thus a means by which to produce valuable prod- ucts while replacing an organic solvent with CO2 (as long-chain aldehydes could only be produced in bulk or in organic solvent). Further, reaction in CO2 will al- low much higher CO and H2 concentrations and hence potentially much faster rates. Indeed, Erkey et al. sus- pected that the high CO and H2 concentrations were potentially the cause for differences in the rate ex- pression between hydroformylation of 1-octene car- ried out in CO2 (using a fluorinated phosphine Rh catalyst) versus that in a conventional liquid. Interest- ingly, Leitner found that internal olefins, which are ‘notoriously unreactive’ in conventional solvents, are hydroformylated with high rates and excellent yields. Erkey examined the effect of ligand structure (most notably, position and nature of the fluorinated pony- tail) on the rate of hydroformylation and found that the activity decreased as the basicity of the ligand de- creased. Hence, increasing the fluorine content of the ligand would tend to enhance the solubility of the cata- lyst in CO2, but decrease the activity. Indeed, increas- ing the fluorine content of the ligand will also increase the cost (both through an increase to molecular weight and the inherent cost of fluorinated compounds). Con- sequently, an optimization problem is created, where increasing fluorine content to the ligand lowers certain capital and operating costs owing to lower required operating pressure, while raising catalyst cost. A pos- sible solution to this problem would be to decouple the effects that create the optimization problem, i.e. find a way to enhance solubility of the catalyst without re- sorting to fluorination. Xiao et al. at the University of Liverpool has examined this route [105], employing carbonyl groups attached to aryl phosphine ligands to enhance catalyst solubility in CO2 . Akgerman et al. have investigated homogeneous hy- droformylation in supercritical CO2 for a number of years [106]. In 1997, Guo and Akgerman reported the homogeneous hydroformylation of propylene in CO2 using a soluble cobalt catalyst. Here, both the rate con- stant and the selectivity were found to be functions of pressure, each increasing significantly as pressure increased from 90 to 190 bar. The apparent effect ofPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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